Automatic gain control circuit



w. R. VKocH 2,082,961

AUTOMATQIC GAIN GONTROL CIRCUIT June 8, 1937.

f Filed July 6, 1934 2 Sheets-Sheet 1 I Ll Z0 f 22921 ff/Mm? 70 mi I ,4M/L

INVENTOR WIN FIELD R. KOCH 70 ,4.146. 'le/A5 BY fm ,9.5 ,4f/0

j: I ATTORNEY June 8, 1937. w, R, KOCH 2,082,961

AUTOMATIC GAINQONTROL CIRCUIT Filed July 6q 1934 2 Sheets-Sheet 2 ,4M/2L' T ir 52) A MMAM 70 40p/0 vvvvv u' INVENTOR VWINFIELD KOCH ATTORNEY Patented June 8, 1937 UNITED STATES PATENT OFFIQE Winfield R. Koch, Camden, N. J., assigner to Radio Corporation of America, a corporation of Delaware Application July 6, 1934, Serial No. 733,987

12 Claims.

My present invention relates to gain control circuits, and more particularly to such circuits when utilized for automatic volume control.

One of the main objects of the present invention is to provide automatic gain control circuits for amplifiers, Whether of high frequency or audible frequency, which circuits embody control tubes having cutput electrodes adapted to be energized by alternating current energy, the

lo alternating current energy being of local oscillation frequency.

Another important object of the present invention is to provide automatic volume control circuits for radio receivers wherein the automatic volume control tubes may have their plates energized by alternating current energy, in place of the usual direct current sources, and the alternating current energy being derived from an existent source in the receiver, such as the tunable o local oscillator circuit of a superheterodyne recelver.

Still other objects of the invention are to improve generally the efficiency of automatic gain control circuits for radio receivers, and more especially to provide in radio receivers automatic volume control circuits, and in some instances interchannel noise suppressor circuits, which are not only reliable inoperation, but readily assembled in a radio receiver.

The novel features which I believe to be characteristic of my invention are set forth in particularity in the appended claims. The invention itself, however, both as to its organization and method of operation, Will best be understood by reference to the following description taken in connection with the drawings, in which I have indicated diagrammatically several circuit organizations whereby my invention may be carried into effect. In the drawings:-

Fig. 1 diagrammatically shows an automatic volume control system embodying one form of the invention,

Fig. 2 shows a modification,

Fig. 3 shows still another modification,

Fig. 4 shows an automatic volume control system embodying an interchannel noise suppressor circuit according to the present invention,

Fig. 5 shows a modified interchannel noise suppressor circuit.

Referring now to the accompanying drawings, wherein like reference characters in the different gures correspond; to similar circuit elements, there is shown in Fig. 1 an automatic volume control system for a superheterodyne receiver 55 particularly. It is to be clearly understood that (Cl. Z50-20) the nature of the receiver with which the volume control network is associated is of comparatively little importance as far as the present application is concerned. The superheterodyne type of receiver has been employed as an illustration because it is at the present time substantially universally employed in broadcast reception in the United States and in many foreign countries. For this reason the various networks of the receiver are conventionally shown in Fig. 1, and it will be observed that the numeral I designates a rst detector upon which is impressed energy of local oscillation frequency from a local oscillator 2. The intermediate frequency output of the network l is impressed upon an intermediate frequency amplifier 3.

rlhe signal input circuit to the first detector may be connected to a signal collector, or to one or more stages of tuned radio frequency amplificaton. The amplified output of network 3 is then impressed upon a second detector, or audio demodulator, through leads 4. The demodulated energy is utilized in one or more stages of audio ampliiication, and then nally reproduced. The receiving system could be employed as a portable receiver' in an automobile, or in a cabinet in the home of the user. The operating range of the receiver can be in the broadcast range, or the receiver may be of the multi-range variety, and in that case it could also be operated in the Various short wave bands outside the broadcast band.

The automatic volume control system is utilized in the receiver, as is well known at the present time, in order to maintain a substantially constant carrier energy level at the input of the audio demodulator. Because of fading phenomena, or other causes which result in effects analogous to fading, the signal energy level at the audio demodulator tends to fluctuate. The automatic volume control system overcomes this tendency, and for this reason is utilized in modern radio receivers. The automatic volume control network shown in Fig. 1 embodies a tube 5 which is of the pentode type, and its resonant input circuit 6 is connected between its signal grid and cathode. The resonant input circuit 6 is coupled to the resonant output circuit of the intermediate frequency network 3, and the coils of both resonant circuits may comprise the primary and secondary windings of the coupling transformer M. y

The voltage source B is employed to provide positive potential for the screen grid of tube 5, and the voltage source C furnishes negative bias for the signal grid of tube 5. The plate of the pentode tube is connected to the low alternating Voltage side of input circuit 6 through a path which includes the secondary winding l' of the transformer M1, lead Band resistor R1, a by-pass condenser C1 being connected in shunt across resistor R1. A resistor R2 is connected to the lead 8, and is arranged in series with coil l, a by-pass condenser C2 being connected in shunt across resistors R1 and R2. The primary winding 9 of transformer M1 has a tuning condenser Il) connected across it.

The circuit S-l is the tunable local oscillator circuit of the local oscillator 2, and the variable condenser I0 is, of course, mechanically uni-controlled with the variable tuning condensers of the rst detector and radio frequency amplifier stages in a manner Well known to those skilled in the art. The symbol M1 has been shown disposed between dotted line arrows coupling coils 9 and 1, and it will be understood that these two coils are magnetically coupled so that local oscillator energy may be impressed upon the coil 1. The physical connection between these two coils may be made in any desired manner, and these coils may be the actual windings of a transformer, ora link circuit may be used to couple them. f

The tube 5 functions as the AVC rectifier, and derives itssignal energy from the output of the intermediate frequency amplifier. For this reason the input circuit B is fixedly tuned to the yoperating intermediate frequency. Instead of using a separate source of direct current voltage for the plate of the tube 5 which produces the control voltage for gain control, the tube is operated with alternating current on the plate. The alternating current is obtained from a source present in the receiver, and it will be observed that this source is the local oscillator. This permits the utilization of common A, B and C battery operation, and no more than the usual voltage need be used. Thus, AVC voltage requirements are simplified, and improved AVC is permitted with a battery operated receiver, such as used in sections where power lines are not available, or in automobiles.

The rectier 5 is normally biased somewhat moi'e negative than is required to reduce the plate current to zero. When a signal is timed in, the intermediate frequency energy impressed on the signal grid of rectier tube 5 will cause the plate currentV to flow, and this will make the plate end of resistor R1 more negative. In turn, this increases the bias on the amplifier tubes to control the amplification of the signal energy. The lead connected to the negative end of'resistor R1 has been designated AVC to denote the direct current gain control connection to -the grids of the Various controlled amplifier i radio Vfrequency amplifier. tubes, the interme- The tube 5, using alternating current on the plate, really functions only during part of the cycle. During the half cycle in which the end of the transformer winding l nearest the plate of vthe tube 5 is negative, no current can flow. During the following half cycle, this end of the winding becomes positive, and current will ow, provided the grid is not too negative, and condenser C1 will be charged up.

The arrangement in Fig. 1 shows an arrangement wherein a series feed is provided for the plate of the AVC rectier tube 5. In Fig. 2 there is shown a similar circuit arrangement differing from that shown in Fig. l only insofar as a shunty voltage feed is provided for the plate of the rectifier 5. It will be observed that in this arrangement a coupling condenser Il is utilized to couple the high alternating voltage side of the tunable local oscillator circuit to the plate circuit of' tube 5. The signal energy for the rectiiier tube is impressed upon its signal grid from the tuned circuit 6 through a coupling condenser i2, and a grid leak resistor I3 is connected between the grid side of condenser i2 and the negative terminal of the voltage source C. It will be observed that connections not necessary to an Yunderstanding of the arrangement are omitted in. order to preserve simplicity of disclosure. The condenser Il is charged up by the plate currents.

Suiicient voltage from the local oscillator circuit may be obtained in a superheterodyne receiver, and by proper design there should be no diculty in obtaining a peak oscillator voltage of 40 to 60 volts. Those skilled in the art will readily be able to design the local oscillator circuit so as to provide a proper operating voltage for the plateof rectifier tube 5.

In Fig. 3 there is shown still another arrangement for utilizing the local oscillator voltage of a superheterodyne receiver as the operating voltage for the plate of the automatic volume control tube. In this arrangement -a small diode, or other rectifier, is added so that the bias on the controlled amplifier tubes can be made twice as high as the peak oscillator voltage. The circuit shown issimilar to that usually employed for voltage doubling, except that the automatic vo1 ume control tube is used in place of one of the diodes, and the connection is similar. The signal strength determines the amount of drop appearing across the automatic volume control resistor R, which furnishes the bias for the grids of the controlled amplifier tubes. The pentode rectiiier 5 has its signal grid connected to the cathode of diode tube il! through a resistor l5, resistor l5 being connected between the cathode and anode of diode l.

The anode side of resistor I is connected to the negative terminal of the voltage source C, and a signal coupling condenser Vl connects the cathode side of resistor l5 to the signal network feeding the second detector. The voltage doubling diode li8 has its cathode connected to the plate of tube 5', which the anode of diode I8 is connected to the cathode of tube 5 through the bias Voltage resistor R, a by-pass condenser i9 beingV connected across the resistor R. The cathode and anode of diode i8 are, additionally, connected by a path which includes the coil 'l' and the condenser 20. The tunable local oscillator circuit 9--16 is magnetically coupled to the coil l', and the condenser 2U is charged by the diode anode Vto have the polarity shown. The function of diode rI4 is to change the signal aosaeei voltage'to a steady direct current voltage for controlling the grid of tube Audio and radio frequency voltages are filtered from the input to this grid by resistor l5 and the condenser I5'. In this way thev AVC is rendered independent of the modulation. s

In Fig. 4 there is shown an interchannel noise suppressor circuit embodying the present invention. In the arrangement shown in Fig. 4 the automatic volume control circuit may be of any well known and conventional type. For this reason the lead designated AVC, and which lead emanatesfrom the second detector network, is to be understood as representing any desired form of volume control circuitwhich maybe used to control the gain of the radio frequency and intermediate frequency amplifier stages. In fact, any of the automatic volume control arrangements shown herein may be utilized for that purpose. Fig. 4 is particularly directed to disclosing the manner of utilizing the local oscillator energy for operating an interchannel noise suppressor, or a muting, network. As is well known to those skilled in the art, such a network is utilized to quiet, or silence, the operation lof the receiver between station positions of the tuning means of the receiver, or, in general,

whenever `the signal input to the receiver fallsv below a predetermined signal threshold value.

In particular, the noise suppressor operates by controlling the action of the audio frequency amplifier tube 2|. The noise suppressor tube is shown as of the type comprising a pentode section and a diode section, and is designated by the reference numeral 22,. Signal energy is impressed upon tube 22 by means of the transformer M3 having its primary `and secondary windings tuned to the operating intermediate frequency, the primary winding being coupled to the intermediate frequency network feeding the audio demodulator, or second detector. The tuned secondary circuit of transformer M3 is connected between the cathode of tube 22 and the diode anode 23 through a resistor 24, the latter being shunted by a radio frequency bypass condenser. The signal grid of the pentode section of tube 22 is connected to the anode side of resistor 24 through a resistor 25, a by-pass condenser being connected between the cathode and the grid side of resistor 25.

The negative terminal of the battery source C is connected to the plate of the pentode section of tube 22 through a path which includes the coil 26, resistor 21, the latter being shunted by condenser 28. The cathode of tube 22 is connected by lead 29 toth-e cathode of the audio amplifier 2|. The signal grid of tube 2| is connected to the plate side of resistor 21 through a path which includes resistor 30 and resistor 3|, a condenser 32 being connected between the cathode of tube 2| and the low alternating voltage side of resistor 30. The tunable local oscil lator circuit is again represented by the numerals 9 and lll, and the coil 26 is magnetically coupled to the coil 9 of the oscillator circuit. The arrangement shown in Fig. 4 shows a series feed for the plate voltage of the noise suppressor tube 22, and also a series feed for the diode of this tube.

In Fig. 5 there is shown a similar arrangement which differs from Fig. 4 only in that -a shunt feed is provided for the plate voltage ofthe noise suppressor tube, and a shunt feed is also provided for the diode section of the tube. In

Fig. 5 the high alternating voltage side of the local oscillator circuit 9-||l is connected by coupling condenser 33 to the plate of the suppressor tube, the network connected to the diode section of the suppressor tube being slightly modified in the manner shown.

- One of the two resistors 22', connected between the diode anode and cathode, serves to let the charge on the coupling condenser leak olf when the signal voltage is removed. The other resistor 24 and the condenser, between grid and cathode, serve as a filter to remove audio and radio frequency voltages from the input to the grid of the tube- In this way the control is rendered independent of the Variation in the carrier envelope caused by the modulation.

The arrangements shown in Figs. 4 and 5 operate in the same manner, and function to maintain the output of the receiver quiet until a signal carrier of at least a predetermined strength is tuned in. It will be noted that in each case the common A, B and C batteries are used for all tubes. In each case the alternating current for the operation of the plate of the suppressor tube is secured from the local oscillator tube. This alternating current on the plate is the same as if .a separate battery were usedby permitting the load resistor to be connected to at the cathode, or to a point more negative than the cathode. In operation, and without any signal present, current isl flowing in the plate circuit of the noise suppressor tube. The voltage drop in resistor 21 will bias off the audio tube to render it inoperative. As soon as a strong signal is tuned in the diode section of tube 22 will bias olf the grid'of the pentode section of tube 22, and cut down the plate current flow through resistor 21. Thus, the voltage drop across resistor 21 will be greatly reduced, and approach zero as a limit, and the audio tube will then be permitted to function normally.

Instead of employing a diode section in connection with the noise suppressor tube, the ordinary grid leak detector operation of the control grid of the pentode section of the tube may be used. Again, a portion of the voltage drop across resistor 21 may be used to partially bias off an intermediate frequency amplifier tube feeding the noise suppressor tube. The action will then possess more of an o-on characteristic, and be less gradual.

While I have indicated and described several systems for` carrying my invention into effect, it will be apparent to one skilled in the art that my invention is by no means limited to the particular organizations shown and described, but that many modifications may be made without departing from the scope of my invention as set forth in the appended claims.

What I claim is:

1. In an automatic gain control system for a superheterodyne receiver, an amplifier whose gain is to be controlled, a rectifier, the rectifier including aV tube provided with a plurality of electrodes, certain of said electrodes comprising input electrodes, means for impressing signals upon `the input electrodes, and means for impressing local oscillations generated in the receiver upon an output electrode of said rectifier tube, a resistor in the space current path of said rectier tube across which is developed a direct current voltage derived from the rectifier space current in response to a predetermined change in signal amplitude, and a gain control connection connected between a negative point on the resistor anda gain control electrode of the am.- plier.

2. In a receiver of the automatic volume con-v trol type,. a demodulator, an audio ampliiier, a background noise suppressor tube, said tube including an amplifier section and a diode section, means for impressing signals upon the diode section, said diode section providing a rectier circuit, means responsive to variations of current in the diode rectier circuitfor varying the space current flow in said amplifier section, a gain consaid output electrodes.

5. In combination in a superheterodyne receiver of the typeincluding a local oscillator netsaid oscillations, means responsive to variations in signal carrier: amplitude for regulating the space current flow of said path,.said last means comprising a pair ofrsignal input electrodes disposed within said rectier tube, and means for applying said control voltage to at least one gain Vcontrol electrode of the amplifier.

6. In combination in a superheterodyne re- Y ceiver of the type includinga local oscillator network and at least one signal amplifier tube, a rectifier tube network having a uni-directional space current path established between at least two electrodes thereof, an impedanceV in said path for developing a gain control voltage from the space current now therethrougn'means. for impressing oscillations from said oscillator. network between said two electrodes forrectication of said "oscillations, means including a signal rectiiier responsive to variations in signal'carrier amplitude for regulating the space current flow of said path, said last rectiiier having its electrodes disposed within the said rectier tube, and means for applying said control voltage vto at least one gain control electrode of the amplifier.

Y In combination in a superheterodyne receiver of. the type including a local oscillator network and at least one audiovsignal amplifier tube, a rectifier network having a uni-directional space current path established between at least two electrodes thereof, an impedance in said path for developing a gain control voltage from the space current' flow therethrough, means for impressing oscillations fromV said oscillator network between said two electrodes for rectification of said oscillations, meansr responsive to variations in signal carrier amplitude for regulating the space current i'low of said path, and means for applying said control voltage to at least one gain control electrode of the amplifier in a sense to Yrender the amplifier inoperative when the signal carrier amplitude decreases below a desired intensity level;

8. In a superheterodyne receiverfof. thetype including'4 a local. oscillator network, a demodulator, and at least one signal transmission tube coupled to the demodulator, means responsive to signal carrier amplitude variations at the receiver input for maintaining the amplitude substantially uniform at the demodulator input circuit, noise suppression means comprising an electron discharge device having an impedance connected in its space current path, means for impressing oscillations from said oscillator network upon two electrodes of the rectifier between which said current flows, direct current connections between said impedance and the input electrodes of said transmission tube for rendering the latter ine'icient with thevoltage developed across the impedance by the rectifier space current in the absence of desired signals, and auxiliary means, responsive to received signals, for reducing the rectifier space current ilow through the impedance.

9. In a superheterodyne receiver of the type including a localoscillator network, a demodulator, and at least one signal transmission tube coupled to the demodulator, means responsive to signal carrier amplitude variations at the receiver input for maintaining the amplitude substantially uniform at the demodulator input circuit, noise .suppression means comprising an electron discharge device having an impedance connected in its space current path, means for impressing oscillations from said oscillator network upon two electrodes of the rectier between which said current flows, YdirectV current connections between said impedance and the input electrodes of said transmission tube for rendering the latter ineicient ,with the voltage developed across the impedance bythe rectifier space current in the absence of desired signals, and auxiliary means including a control electrode disposed in said path, responsive to received signals', for` reducing thev rectier space current ow through the irnpedance.

l0. In a superheterodyne receiver of the type including a local oscillator network, a demodulator', and at least one signal transmission tube coupled to the demodulator, means responsive to signal carrier amplitude variations at the receiver inputfor maintaining the amplitude'substantiallyuni'form at the demodulator input circuit, noise suppression means comprising an electron discharge device having an impedance connectedin its spacecurrent path, means for impressing oscillations from said oscillator network upon; two electrodes of the rectifier between which said current flows, direct current connections between said impedance and the input electrodes of said transmission tube for rendering the latter inefficient with the'voltage developed across the impedance by the rectifier space current in the absence of desired signals, and auxiliary means, responsive to received signals, for reducing the rectifier space current flow through the impedance, said auxiliary means comprising a dioderectier coupled to the demodulator input circuit, and a direct current control connection from the diode to a control electrode disposed in said first rectifier' space current path.

11. In a superheterodyne receiver of the type including a local oscillator network, a demodulator, and at least one signal transmission tube coupledzto the demodulator, means responsive to signal carrierv amplitude variations at the receiverV input for'maintaining the amplitude substantially unifornrat the demodulator input circuit,.noise;suppression meansicomprising an electron discharge device having an impedance connected in its space current path, means for irnpressing oscillations from said oscillator network upon two electrodes of the rectifier between 5 which said current flows, direct current connections between said impedance and the input electrodes of said transmission tube for rendering the latter inei-Hcient with the voltage developed across the impedance by the rectifier space curl0 rent in the absence of desired signals, and auxiliary means, responsive to received signals, for reducing the rectifier space current flow through the impedance, said transmission tube following said demodulator and receiving therefrom the 15 audio component of demodulated signals, and the auxiliary means including a rectifier coupled to the demodulator input circuit to have intermediate frequency energy impressed thereon.

12. In combinationin a superheterodyne re- 20 ceiver of the type including a local oscillator network and at least one signal amplier tube, a rectifier tube network having a uni-directional space current path established between at least two electrodes thereof, an impedance in said path for developing a gain control voltage from the space current ow therethrough, means for impressing oscillations from said oscillator network between said two electrodes for rectification of said oscillations, means responsive to variations in signal carrier amplitude for regulating the space current ow of said path, and means for applying said control voltage to at least one gain control electrode of the amplifier, said second means comprising a signal rectifier whose electrodes are disposed in said rectier tube, anda control electrode disposed in said space current path and having a direct current connection to an electrode of said second rectier.

WINF'IELD R. KOCH. 

